Linux

Gnome 3, despite being simple by design and relatively easy for us techy folk to adapt to, is likely still quite a radical departure for someone who understands absolutely nothing about computers besides what they had to learn to do their jobs.

Explaining Computers did a good video on Zorin, where he shows it can be more Windows-like. Linux Mint is another good option, but it wouldn't be too different from the Windows themed KDE you tried, IMO.

I think with people such as you described, where change is feared so much, it probably is best to stick with the most familiar possible layout, and to not share more information about the differences than absolutely needed. Perhaps the initial Gnome 3 shock made them shut down even when KDE was introduced instead? Hard to say. Good on ya for trying, though.